The present invention relates generally to preparation of semiconductor wafers. More specifically, the present invention relates to a modified susceptor to decrease autodoping of semiconductor wafers when the wafers are being treated in a barrel-type reactor.
Chemical vapor deposition is a process for growing a thin layer of material on a semiconductor wafer so that the lattice structure is identical to that of the wafer. Using this process, a layer having different conductivity may be applied to the semiconductor wafer to achieve the necessary electrical properties. Chemical vapor deposition is widely used in semiconductor wafer production to build up epitaxial layers such that devices can be fabricated directly on the epitaxial layer. For example, a lightly doped epitaxial layer deposited over a heavily doped substrate permits a CMOS device to be optimized for latch up immunity as a result of the low resistance of the substrate. Other advantages, such as precise control of the dopant concentration profile and freedom from oxygen are also achieved. Conventional epitaxial deposition processes are disclosed in U.S. Pat. Nos. 5,904,769 and 5,769,942, which are herein incorporated by reference.
The epitaxial deposition process takes place in a reactor. There are various types of reactors including horizontal reactors, vertical reactors (also called pancake reactors) and barrel reactors. The barrel reactor (also referred to as a batch reactor) is advantageous because it simultaneously processes a number of wafers, typically between 10 and 15 wafers, in a single run. In a barrel reactor, a barrel susceptor is used to support the wafers during the epitaxial deposition process. The silicon wafers are loaded onto the barrel susceptor, and then the susceptor is loaded into a reaction chamber of the reactor. FIG. 1 illustrates a typical conventional barrel susceptor. The barrel susceptor has generally laterally facing, sloping faces, and each face has a pair of circular recesses aligned in a generally vertical column for receiving a single wafer. Each recess of the susceptor has a solid, non-porous rear surface on which a backside of the wafer rests. Preferably, the body of the susceptor is constructed of silicon carbide coated graphite to reduce the amount of contaminants such as iron released from the graphite into the surrounding ambient during high temperature processes, although other material used in manufacture of the body is contemplated and within the scope of this invention. Conventional barrel susceptors are well known in the art and described in U.S. Pat. No. 6,129,048.
The epitaxial deposition process typically involves two steps. First, after the wafers have been loaded onto the barrel susceptor and the susceptor is received in the reaction chamber, the front surfaces of the wafers are subjected to a cleaning gas such as hydrogen or a hydrogen/hydrochloric acid mixture at about 1150° C. to “pre-bake” and clean the front surfaces of the silicon wafers and remove any native oxide on the surfaces to allow the epitaxial silicon layers to grow continuously and evenly onto the front surfaces. In the second step of the epitaxial deposition process, the front surfaces of the wafers are subjected to a Si source reactant gas. For example, a vaporous silicon source such as silane or trichlorosilane may be introduced at about 800° C. or higher to deposit and grow epitaxial layers of silicon on the front surfaces of the wafers. During both steps, the susceptor is rotating within the reaction chamber.
A problem encountered during the high temperature pre-bake and high temperature growth of the epitaxial silicon layer is the out-diffusion of dopant atoms such as boron or phosphorus through the back surface of the silicon wafer. With conventional barrel susceptors, the dopant atoms that out-diffuse from the back surface can effuse from between the wafer edge and the susceptor toward the front surface of the wafer. These dopant atoms can be incorporated into and contaminate the growing deposition layer and degrade the resistivity uniformity near the wafer edge. Silicon wafers having etched or polished back surfaces are subject to out-diffusion of dopant atoms from the back surface during the epitaxial deposition process which can lead to unwanted autodoping of the front surface.
Several methods have been suggested for attempting to eliminate autodoping. Hoshi (Japanese Unexamined Patent Application No. JP11-87250) discloses using vacuum sucking on the edge of a susceptor to evacuate boron dopant on the edge of the susceptor and prevent autodoping. This process may affect wafer edge uniformity and thickness and requires substantial modification to existing epitaxial deposition systems. Nakamura (Japanese Unexamined Patent Application JP10-223545) discloses a modified susceptor having slots on the edge of the susceptor such that the out-diffused dopant atoms would be pushed down through the slots and into the exhaust. This method also allows a substantial amount of the deposition gas to be evacuated below the back surface of the wafer which can lead to premature corrosion of the exhaust system and safety concerns.